This invention is directed to an improved method for the dewatering of waste solids generated in mineral processing operations on mechanical filter or separation devices. In processes of this type, solids are typically treated to concentrate them, using mechanical means which are assisted by the application of water soluble coagulants and flocculants. Such materials such as thickened coal refuse slurry solids; thickened copper ore refuse slurries; precious metals refuse slurries; taconite refuse slurries; trona refuse underflow slurries; titania refuse underflow slurries; sand and clay refuse generated from the mining, crushing and grinding of construction materials; clay slurries; and wastes from the treatment of bauxite must be concentrated and dewatered prior to disposal or other disposition of such wastes. Often, these materials contain as little as 0.5% solids to 20% solids. These materials may have undergone initial treatment, such as is generally the case in dealing with coal and copper ore refuse slurries, to bring the concentration of solids to 20% to 35% by weight.
The normal treatment for these types of concentrated wastes is to mechanically dewater such slurries with the aid of coagulants and flocculants. Often, the concentrated slurries while being subjected to mechanical dewatering are first treated with a flocculant, generally a high molecular weight anionic material, followed by the application of a coagulating amount of a water-soluble cationic coagulant material.
The typical equipment used for mineral solids dewatering includes twin belt press, disc, gravity, vacuum, rotary table (Bird), sand, drum, string, and plate and frame filters. However, one of the most prominent means of dewatering waste mineral solids involves the use of the twin belt press.
The twin belt press is a filtration device that uses a combination of gravity and pressure dewatering. These are four basic operational stages in a twin belt press. (1) Pretreatment of the slurry, (2) Gravity drainage of free water, (free drainage zone) (3) Wedge zone, and (4) High pressure zone (S-rolls).
Good chemical conditioning is the key to successful and consistent performance of the belt press, as it is for other dewatering processes. In the pretreatment stage, the slurry is treated with chemicals which increase the free water and stabilize the slurry so it stays on the belt. As the slurry is fed onto the filter media, the formation of a uniform evenly-distributed slurry is essential to successful operation of the free drainage, wedge, and pressure zones.
The gravity stage allows free drainage of the water to the point where pressure can be applied to the slurry. Failure to remove the free water in the gravity zone will result in a cake that extrudes (squeezes) off the press as pressure is applied. In the wedge zone, the pressure applied to the cake is gradually increased, further stabilizing the slurry in preparation for the high pressure zone. The cake is then wrapped around a series of S-rolls. The radius of each S-roll is progressively smaller, hence greater pressure, causing increased water release and allowing greater compaction of the cake. The tension of the belt also affects the applied pressures in the high pressure zone. Cake discharge is accomplished over a discharge roller assisted by a discharge blade. Failure to sufficiently dewater the slurry at any stage can result in a fluid cake which is expelled off the sides of the belts.
Twin belt filter presses are often used to dewater solids resulting from the processing of mining waste solids which term includes, in some instances, solid separation in the purification of ores. Mining solids from such mining operations as copper ore processing, phosphate rock purification, uranium processing and the like often are dewatered on twin belt filter presses. A particularly important area of mining where twin belt filter presses are used is in the dewatering of coal refuse solids. To improve drainage and reduce high pressure zones, it is common practice in the utilization of twin belt filter presses to first treat the solid suspensions prior to filtration on the twin belt filter press with a flocculant followed by a coagulant. This treatment is often used in conjunction with coal refuse slurries prior to filtration on a twin belt press. A coagulant capable of improving the operational efficiency of twin belt filter presses, particularly in the dewatering of coal refuse solids, would represent a worthwhile advance in the art.
Although some inorganic materials, principally alum and iron salts, are still used as coagulants, water-soluble organic polymers are now commonly used as coagulants. Both naturally occurring and synthetic polymers find use as coagulants and flocculants in the mining industry. The principal natural polymers are starch and guar, both of which are high-molecular weight polymers of simple sugars (i.e,. polysaccharides). Starch is a polymer of glucose consisting of a mixture of linear (amylose) and branched (amylopectin) segments.
Synthetic polymers are advantageous in that they can be tailored to a specific application. Therefore, there is now a wide range of commercially available polymeric coagulants and flocculants of varying charge, composition and molecular weight. The most widely used synthetic coagulants are polydiallyldimethyl ammonium chloride as described in U.S. Pat. No. 2,926,161 and condensation polymers of dimethylamine and epichlorohydrin such as those described in U.S. Pat. Nos. Re. 28,807 and Re. 28,808. These polymers vary greatly in molecular weight, typically ranging from several thousand to as high as 100,000. Condensation polymers are made in solution form, and are available commercially as aqueous solutions containing 1-20 weight percent polymer. Polydiallyldimethyl ammonium chloride is a vinyl addition polymer, which, at the molecular weights used for coagulation, has also been made in solution form. Typical commercially available polydiallyldimethyl ammonium chloride is available in aqueous solutions containing 1-20% by weight polymer.
Dry water-soluble polymers such as dry polydiallyldimethyl ammonium chloride have also been used to dewater coal refuse slurries on twin belt presses. These polymers have met with some success, but to be successful in twin belt and other mechanical dewatering applications, must be first dissolved in water prior to using. Disadvantages of dry polymer are that it produces dust; if not carefully fed, may produce gelled agglomerates which can foul feeding equipment; and is difficult to handle, in that bags of the material must be moved into proximity of the thickener. The polymers of the present invention overcome these deficiencies while providing activities equivalent to or better than those attained using dry polymers.